Amphidromous fishes are hatched in freshwater streams and float to the ocean as larvae. After an oceanic pelagic larval phase they transition to the postlarval phase, and migrate back into freshwater where they grow to adults and spawn. Previous research has shown that Sirajo Gobies undergo synchronous mass migrations from the ocean to freshwater during limited periods of the lunar cycle. Postlarvae are harvested in artisanal fisheries during migration episodes and could be a resource subsidy for estuarine and freshwater environments. As such, the magnitude and diel pattern of these migrations has critical implications for the estuarine and riverine ecology of tropical islands and the management of their fisheries. Our first objective was to determine the diel-scale recruitment phenology of postlarval Sirajo Gobies in the Río Grande de Arecibo, Puerto Rico. We accomplished this objective by intensively sampling ingressing postlarvae at the Río Grande de Arecibo river mouth during a four-day mass migration episode in July 2015. We then fit a generalized additive model (GAM) of Sirajo Goby ingress over time and used it to estimate the total abundance and biomass of postlarval Sirajo Gobies that migrated into the Río Grande de Arecibo during the migration episode.
Results/Conclusions
We observed that peaks in postlarval ingress occurred from midnight to 0600 with maximum mean ingress between the hours of midnight and 0300. These results indicate that Sirajo Gobies migrate to river mouths as pulses of very dense groups of individuals, and that these pulses generally occur at night. These migration patterns could be behavioral adaptations to reduce predation on the Sirajo Gobies. Migrating in large groups would increase an individual’s chances of survival through a predator swamping mechanism, and nocturnal migrations could reduce the effectiveness of sight feeding predators. Our model estimated that 21,513,418 Sirajo Gobies migrated through the Río Grande de Arecibo over four days. This abundance estimate represents a biomass of 1,908 kg of Sirajo Gobies. This large biomass estimate demonstrates that postlarval amphidromous fishes represent a significant resource subsidy for estuarine and riverine systems during specific cyclic time periods. The biomass and abundance estimates presented here are useful for setting harvest quotas that will ensure the sustainability of this important resource.